FIG. 1 is a diagram showing a heterogeneous-network wireless communication system 100 including a macro base station and a micro base station. In the present specification, the term heterogeneous network means a network in which the macro base station 110 and micro base stations 121 and 122 coexist although the same radio access technology (RAT) is used.
The macro base station 110 has wide coverage and high transmit power and means a general base station of a wireless communication system. The macro base station 110 may be referred to as a macro cell.
Each micro base station 121 or 122 may be referred to as a micro cell, a pico cell, a femto cell, a home eNB (HeNB), a relay, etc., for example. The micro base station 121 or 122 is a small version of the macro base station 110 and may independently operate while performing most functions of the macro base station. The micro base station may be an overlay type base station mounted in a region covered by the macro base station or may be a non-overlay type base station mounted in a shadow region which is not covered by the macro base station. The micro base station 121 or 122 may have narrower coverage and lower transmit power and accommodate fewer user equipments (UEs) as compared to the macro base station 110.
A UE 131 (hereinafter, referred to as a macro UE) may be directly served by the macro base station 110 and a UE 132 (hereinafter, referred to as a micro UE) may be served by the micro base station 122. In some cases, the UE 132 located in coverage of the micro base station 122 may be served by the macro base station 110.
The micro base station may be classified into two types depending on whether UE access is restricted. A first type is a closed subscriber group (CSG) micro base station and a second type is an open subscriber group (OSG) micro base station. The CSG micro base station may serve only authorized UEs and the OSG micro base station may serve all UEs.